This invention relates generally to gate circuits for gate turn-off (GTO) thyristors and more particularly to a gate circuit for a GTO thyristor which uses two power sources for supplying currents to the gate electrode of the GTO thyristor and which can minimize power loss in the gate circuit.
The GTO thyristor is a switching element which is turned on, like thyristors of the other types, with gate current (gate turn-on current) entering into the gate electrode and turned off, unlike the other types, with reverse gate current (gate turn-off current) parting from the gate electrode.
The gate controlled turn-on time of the GTO thyristor depends on the magnitude of the gate turn-on current, and the greater the latter, the shorter the former becomes whereas the smaller the latter, the longer the former becomes. When examining a great number of GTO thyristors to make clear the relation between the gate controlled turn-on time and the magnitude of gate turn-on current, that is, turn-on characteristics, the turn-on characteristics prove to be irregular in respect of each of the GTO thyristors. The greater the magnitude of gate turn-on current, the smaller the irregularity in turn-on characteristics becomes whereas the smaller the former, the greater the latter becomes. Incidentally, it is thought in view of electrical equivalency that a GTO thyristor of large capacity consists of a great number of GTO thyristor elements of small capacity which are connected in parallel. In other words, the turn-on characteristics of the internal thyristor elements have tendency to be different from each other. If the irregularity in the turn-on characteristics is large between the internal thyristor elements, current will be concentrated to a certain internal thyristor element which will have been turned on rapidly and the GTO thyristor will face danger of being thermally broken down. Therefore, when turning on the GTO thyristor, it is necesary to supply a large amount of gate turn-on current in order to minimize the irregularity in the turn-on characteristics between the internal thyristor elements.
After the GTO thyristor has been turned on, there is the need of keeping continuous flow of a sufficient gate turn-on current for maintaining the the steady conduction state of the GTO thyristor during the desired conduction period, when taking into consideration such conditions as the GTO thyristor requires a larger holding current than ordinary thyristors and has its major application to a device which is loaded with a load of lagging power factor.
The gate turn-off time of the GTO tyhristor depends on the rising rate (di/dt) of the gate turn-off current, and the greater the latter (di/dt), the shorter the former becomes whereas the smaller the latter, the longer the former becomes. When examining a great number of GTO thyristors to make clear the relation between the gate turn-on time and the rising rate di/dt (namely, the turn-off characteristics), the turn-off characteristics, like the turn-on characteristics, prove to be irregular in respect of each of the GTO thyristors. The greater the rising rate di/dt, the smaller the irregularity in turn-off characteristics becomes whereas the smaller the former, the greater the latter becomes. If the irregularity in the turn-off characteristics is large between the internal GTO thyristor elements, current will be concentrated to a certain internal thyristor element which will have been turned off late and the GTO thyristor will face danger of being thermally broken down. Therefore, when turning off the GTO thyristor, it is necessary to supply a gate turn-off current of high rising rate in order to minimize the irregularity in the turn-off characteristics.
It will be understood from the above description what waveform the gate current suitable for driving the GTO thyristor must have. It will thus be appreciated that there is an urgent demand for advent of a gate circuit which is suitable for producing the gate current of the desired waveform.